Conventional headphones do not account for the effects of the human outer ear to produce realistic sounds and typically make no attempt to produce the sound cues needed to locate the direction of the sound. In a theatre type application, the sound signals from surround sound are created to be played on speakers that are in specific locations in the room. For example, one signal is created to be played on a speaker that is in front of the listener and about 45 degrees to the left. Another is created to be played on a speaker that is about the same distance from the listener but behind the listener and at approximately 60 degrees from directly behind the listener. The angle and distance from the listener creates the correct timing, intensity, pinna effect and head tracking effects to create the intended results. These effects are difficult to reproduce in headsets, due to the limited space that is available for speaker placement, and the need for compact packaging. Further, in conventional headphone design, if the speaker is not placed within a few millimeters of the ear, then the sound quality greatly diminishes.
To provide the effects of surround sound, the normal methods that humans use to locate the direction of sound have to replicated. There are four main methods that humans use, all of which are used in concert as no one method alone is adequate to pinpoint the source of the sound.    1. The time difference of the sound reaching each ear. If the sound is directly ahead or behind the listener, there is no sound difference between the left and right ears receiving the sounds. If the sound is directly to the right of the listener (at 90 degrees from the front of the head) the right ear hears the sound approximately 0.5 ms before the left ear. So, any angle between 0 and 90 creates a unique timing difference. It does not however, distinguish between the angle in front of the ear and the same angle to the rear of the ear. Furthermore, the time difference between the ears also creates a phase difference between the signals at each ear. The time difference is the same for all frequencies but since the wavelengths vary, the phase varies with frequency.    2. Intensity difference. Sounds on one side of the head are louder on that side of the head. High frequency sounds are blocked by the head more than the low frequency sounds so the quality of the sound is altered to the ear opposite the sound source.    3. Pinna effect. The pinna is the outer ear. Sounds coming from the front of the head are reflected by the outer ear to the ear canal. Some of the sound frequencies are reflected more efficiently than others, depending on the ear size and shape, and depending on the direction of the sound. This is the main method for distinguishing between front and rear located sounds. Rear sounds are somewhat blocked by the outer ear and are muffled, front sounds have some of the higher frequencies amplified and sound ‘crisper’. Since ear shape and size is unique to each person (and for each person may even be different on left and right sides) the frequencies being amplified are different for each person.    4. Head tracking. Any remaining ambiguity in the use of the above methods is greatly reduced by the person rotating the head. A turn of the head changes the angle of the sound relative to the ears and so all of the above cues, timing, intensity and pinna effect also change. The change in the cues then gives the brain a second point of view of the sound location, and greatly helps to pinpoint the sound. As little as a 5 degree turn of the head can be enough to fully pinpoint the direction of the sound.
The prior art for surround sound in headsets can be described to be in one of two categories.
The first category includes techniques where physical modifications to the locations of speakers are used to create surround sound, and in some cases the speakers are connected with tubes. Horn tubes are well known, but have not previously been applied to headsets. The horn tubes make the transmission of the sound more efficient and reduce the need for added amplification.
The second category includes techniques that use one speaker at each ear and use electronic methods to alter the signals intended for the speakers to create a virtual surround sound. The methods are called Head-Related Transfer Functions (HRTF); The HRTF alter the timing and intensity of the signals as described in the first two methods above. They also modify the sound so as to mimic the pinna effect. However, since every person has a unique shape to their ears (purportedly as unique as fingerprints), the way that each person has learned to detect the differences due to location are also unique to each person. The HRTF methods attempt to create a modification to sound that many people recognize as their sound cues. However, the methods can not adapt to every person and so are limited. Other prior art describes the use of head tracking in headsets and uses a device (e.g., gyro) to track the rotation of the head and to alter the signals to the speakers to reflect the head rotation. This requires a processor to modify the signals to replicate the desired effect, while in the present invention the signals are left unchanged.
Accordingly, it is a object of the present invention to provide a surround sound headset that overcomes the disadvantages of the prior art. It should produce sounds with such timing, intensity and pinna effects as a listener would expect to hear naturally, no matter the shape and size of the listener's pinna.